Abacavir is the International Nonproprietary Name (INN) of {(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-cyclopent-2-enyl}methanol and CAS No. 136470-78-5. Abacavir and therapeutically acceptable salts thereof, in particular the hemisulfate salt, are well-known as potent selective inhibitors of HIV-1 and HIV-2, and can be used in the treatment of human immunodeficiency virus (HIV) infection.
The structure of abacavir corresponds to formula (I):

EP 434450-A discloses certain 9-substituted-2-aminopurines including abacavir and its salts, methods for their preparation, and pharmaceutical compositions using these compounds.
Different preparation processes of abacavir are known in the art. In some of them abacavir is obtained starting from an appropriate pyrimidine compound, coupling it with a sugar analogue residue, followed by a cyclisation to form the imidazole ring and a final introduction of the cyclopropylamino group at the 6 position of the purine ring.
According to the teachings of EP 434450-A, the abacavir base is finally isolated by trituration using acetonitrile (ACN) or by chromatography, and subsequently it can be transformed to a salt of abacavir by reaction with the corresponding acid. Such isolation methods (trituration and chromatography) usually are limited to laboratory scale because they are not appropriate for industrial use. Furthermore, the isolation of the abacavir base by trituration using acetonitrile gives a gummy solid (Example 7) and the isolation by chromatography (eluted from methanol/ethyl acetate) yields a solid foam (Example 19 or 28).
Other documents also describe the isolation of abacavir by trituration or chromatography, but always a gummy solid or solid foam is obtained (cf. WO 9921861 and EP 741710-A), which would be difficult to operate on industrial scale.
WO 9852949 describes the preparation of abacavir which is isolated from acetone. According to this document the manufacture of the abacavir free base produces an amorphous solid which traps solvents and is, therefore, unsuitable for large scale purification, or for formulation, without additional purification procedures (cf. page 1 of WO 9852949). Other documents also describe the obtention of abacavir from acetone (cf. Susan M. Daluge et al., Nucleosides, nucleotides and nucleic acids 2000, vol. 19, pp. 297-327; WO 9939691 or WO 2008037760). In the last one the preparation of abacavir from ethyl acetate is also described. In some of these documents it is mentioned that the abacavir obtained is a solid but nothing is said about the fact that the compound obtained could be amorphous or could trap solvent in its lattice. The reproduction of the preparation of abacavir from acetone showed that an acetone solvate of abacavir is obtained, which is in agreement with WO 9852949 regarding to the fact that the product traps solvent. The crystallization of abacavir from ethyl acetate as described in WO 2008037760 showed that an ethyl acetate solvate of abacavir is obtained which spontaneously changed to give a brown gum after few hours.
It is not acceptable to formulate pharmaceuticals containing substantial amounts of organic solvent due to potential solvent toxicity to the recipient thereof and changes in potency of the pharmaceutical as a function of the solvent.
In WO 9852949 the use of a salt of abacavir is proposed, in particular the hemisulfate salt which shows improved physical properties regarding to the abacavir base known in the art. Said properties allow the manufacture of the salt on industrial scale, and in particular its use for the preparation of pharmaceutical formulations. However, the preparation of a salt of abacavir involves an extra processing step of preparing the salt, increasing the cost and the time to manufacture the compound. Generally, the abacavir free base is the precursor compound for the preparation of the salt. Thus, depending on the preparation process used for the preparation of the salt, the isolation step of the abacavir free base must also be done.
Thus, there is still a need to find new solid forms of abacavir base suitable to operate on industrial scale, either to be used directly as pharmaceutical active ingredient in a pharmaceutical formulation, or if desired, to be subsequently transformed into a pharmaceutically acceptable salt thereof.
Furthermore, the different solid forms of a pharmaceutically active ingredient can have different characteristics, and offer certain advantages, in methods of manufacture and also in pharmacology. Thus, the discovery of new solid forms can contribute to clear improvements in the efficiency of methods of production and/or improvements in the characteristics of the pharmaceutical formulations of the active ingredients, since some forms are more adequate for one type of formulation, and other forms for other different formulations.